In　this　paper,　the　cyclic　variation　characteristics　of　a　hydrogen-enriched　gasoline　engine　under　various　operating　conditions　were　experimentally　investigated.　The　test　was　carried　out　on　a　modified　four-cylinder　gasoline　engine　equipped　with　an　electronically　controlled　hydrogen　injection　system.　A　hybrid　electronic　control　unit　was　developed　to　govern　the　injection　timings　and　durations　of　hydrogen　and　gasoline　to　accomplish　the　on-line　adjusting　of　the　hydrogen　blending　level　and　excess　air　ratio.　The　engine　was　first　run　at　idle　condition　with　an　idle　speed　of　790　rpm　and　then　operated　at　1400　rpm　to　investigate　the　cyclic　variation　in　a　hydrogen-blended　gasoline　engine　at　different　hydrogen　volume　fractions　in　the　total　intake,　excess　air　ratios,　spark　timings　and　manifolds　absolute　pressures.　The　test　results　demonstrated　that　the　coefficient　of　variation　in　indicated　mean　effective　pressure　was　distinctly　decreased　with　the　increase　of　hydrogen　blending　ratio.　At　1400　rpm　and　a　manifolds　absolute　pressure　of　61.5　kPa,　the　relevant　excess　air　ratio　for　the　engine　lean　burn　limit　was　extended　from　1.45　to　2.55　when　the　hydrogen　volume　fraction　in　the　intake　was　raised　from　0%　to　4.5%.　Besides,　for　a　specified　hydrogen　addition　level,　the　coefficient　of　variation　in　indicated　mean　effective　pressure　was　continuously　increased　but　the　coefficient　of　variation　in　the　peak　cylinder　pressure　was　first　raised　and　then　decreased　with　the　increase　of　excess　air　ratio.　The　experimental　results　also　showed　that　hydrogen　addition　was　more　effective　on　reducing　engine　cyclic　variation　at　low　loads　rather　than　at　high　loads.　Copyright　(C)　2011,　Hydrogen　Energy　Publications,　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.